When a projector is used to give a presentation, the color of images may be affected by ambient light, color drift in the projector light source, or the condition of the screen on which images are projected. Although chromatic adaptation in human vision generally works well to maintain constant perceived color under different ambient light conditions, there are times when the color temperature of ambient light, reflectance of the screen, and other factors result in noticeable color shifts. One may want to avoid color shifts for various reasons. For example, a presentation may involve color as a topic of technical or artistic discussion. In addition, companies are usually deeply concerned about the color of their logos, so a person making a presentation to or on behalf of a particular company would want the logo to have the proper color. Further, a presentation may include images of people, so it is important that skin tone be displayed properly to avoid any distraction from the message of the presentation. In general, ensuring that colors are displayed properly helps to make an effective presentation.
Various approaches have been taken to compensate for color shifts, but those approaches have drawbacks. For example, when a light source is used to pass light through a digital image in a projector, color ratios in the light source may be adjusted to compensate for color shifts. However, color adjustment of the light source alone may not be enough to eliminate color shift. Also, the light source may have degraded over time, becoming another source of color shift. If so, the light source cannot be relied upon to produce colors with accuracy. In another example, as disclosed in JP 2010-119017 A, a calibration chart of black, white and various colors is displayed prior to the presentation. Color ratios in the original document image are adjusted based on a photograph of the calibration chart, and then the adjusted document image is used by the projector. However, ambient conditions may change during the course of a presentation, and it is not practical to display the calibration chart during the middle of the presentation. In a formal setting, the presenter may prefer that meeting attendees not see a calibration chart. Although calibration could be performed earlier before attendees arrive, there is a risk that ambient conditions may change significantly by the time the presentation starts, such as may occur after sunset in a conference room with windows. In general, a one-time calibration approach would fail to compensate for color shifts that can sometimes occur after calibration. Moreover, calibration based on a predefined target requires an extra step that users may prefer to avoid. Accordingly, there is a need for a way to correct color shifts during a presentation.